In most recent patents involving the pressure leaching of copper materials, copper is recovered from solution by solvent extraction and electrowinning. This is recognized as an improvement, with regards to reagent costs and product purity compared to earlier patents that use cementation with iron to recover copper (see for example, U.S. Pat. No. 2,686,114 by Chemico; and U.S. Pat. No. 4,610,723 to Nogueira et al.).
Most patents that include the pressure leaching of copper materials, including the entire series of U.S. Patents by Phelps Dodge/Freeport McMoran, listed below, do not mention or address the solubility or deportment of arsenic within leaching, letdown / conditioning, or other subsequent steps (see U.S. Pat. Nos. 6,451,089; 6,451,089; 6,497,745; 6,626,979; 6,663,689; 6,680,034; 6,890,371; 6,893,482; 6,972,107; 7,041,152; 7,125,436; 7,341,700; 7,462,272; 7,473,413; 7,476,308; 7,485,216; 7,517,384; 7,666,371; 7,736,487; and 7,736,488).
In general, patents that do address the deportment of arsenic teach the following methods for treating soluble arsenic, either in the pressure leach solution or in the raffinate from solvent extraction:                i. Atmospheric neutralization and precipitation of calcium and/or iron arsenate solids for disposal (Chemico, U.S. Pat. No. 2,686,114);        ii. Evaporation, neutralization and recycle of arsenical solutions to the autoclave (CESL, U.S. Pat. No. 6,755,891); and        iii. Neutralization and precipitation of calcium and/or iron arsenate solids and recycle to the autoclave (Dundee Precious Metals Inc., U.S. Published Patent Applications 2009/0293680 and 2009/0019970, now abandoned).        
Disadvantages to these methods are many. With the recycle of solids or solutions to the autoclave, there is a recirculating load of arsenic to the autoclave. This means that a portion of the iron in the feed is required to react with this recycled material to form iron-arsenic compounds. The higher the arsenic recycle, the less “fresh” arsenic that can be processed to maintain a certain Fe:As molar ratio (as is proposed in both the Dundee and CESL patent documents). None of the above patent references indicate or address the size of this recycle, nor provisions to control or maintain it at certain levels.
Atmospheric precipitation of arsenic with calcium or iron, unless arsenic is precipitated as scorodite (FeAsO4:2H2O), which is not presented in any of the above patent references, will result in the production of a residue that is not likely to be as environmentally stable as materials produced at elevated temperatures.
All of these methods involve the neutralization of solutions that typically contain significant amounts of free acid. In each case, additional neutralizing agent is required to neutralize the free acid prior to recycle or disposal, and, with the use of calcium-based reagents, as is proposed in the patent references, this can result in the loss of the sulphate associated with the free acid, and the sulphate associated with other metal ions that are precipitated, from those solutions as gypsum.
None of the above patents teach methods to minimize or control the amount of arsenic in the pressure leach or raffinate solutions.
In gold pressure oxidation, arsenic in solution is generally dealt with, along with other dissolved metal ions, by neutralization and precipitation with lime and limestone to allow for recycle of the neutralized solution within the pressure oxidation plant, see for example U.S. Pat. No. 4,571,264 to Sherritt Gordon Mines Ltd., now Sherritt International Corporation. Without a need to recover copper or other metals from solution, these gold processes do not otherwise address minimizing or controlling the arsenic in solution.
Most of the patent references involving pressure leaching of gold or copper concentrates address the need to lower the pressure and temperature of the autoclave slurry, prior to subsequent downstream steps, including liquid solid separation (e.g. CCD-countercurrent decantation wash), neutralization or solvent extraction steps. Flashing of the slurry to evaporatively cool the slurry in a flash vessel is proposed in an early patent, U.S. Pat. No. 2,686,114, and indirect slurry cooling (e.g. with slurry coolers or heat exchangers) is proposed in several patents, as summarized in the below-listed patents.                U.S. Pat. No. 2,686,114 to Chemico—flashing;        U.S. Pat. No. 3,962,402 to Freeport Minerals—cooling coils, batch testing;        U.S. Pat. No. 5,071,477 to Barrick Gold—flashing, indirect cooling;        U.S. Pat. No. 5,458,866 to Santa Fe Pacific Gold Corp.—cooling, unspecified;        U.S. Pat. No. 5,698,170 to Placer Dome, now Barrick—flashing;        U.S. Pat. No. 5,895,653 to Placer Dome, now Barrick—flashing;        U.S. Pat. No. 6,251,163 to Placer Dome, now Barrick—flashing, cooling in CCD, or cooling by dilution with wash water in the CCD;        U.S. Pat. No. 6,451,088 to Freeport McMoran—flashing;        U.S. Pat. No. 6,680,034 to Freeport McMoran—flashing, indirect cooling;        U.S. Pat. No. 7,713,500 to Western Minerals—multistage flash cooling;        U.S. Published Application 2009/0293680 to Dundee Precious Metals—flashing; and        U.S. Published Application 2009/0019970 to Dundee Precious Metals—flashing.        
The above-listed U.S. patents to Freeport McMoran/Phelps Dodge dating from 2002 to 2010 use a “conditioning” step. For example, U.S. Pat. No. 7,736,487 indicates that a copper-bearing solution may be prepared and conditioned for metal recovery through one or more chemical and/or physical processing steps. The product stream from a reactive processing step may be conditioned to adjust the composition, component concentrations, solids content, volume, temperature, pressure, and/or other physical and/or chemical parameters to desired values and thus to form a suitable copper-bearing solution.
The teachings of those patents for “conditioning”, in addition to flashing and/or slurry cooling to reduce the temperature of the autoclave slurry, are as follows, with the first patent in this series to mention the specific unit operation noted in parentheses:                i. liquid solid separation (thickening or filtration) (U.S. Pat. No. 6,451,088);        ii. solvent extraction for copper recovery (U.S. Pat. No. 6,451,088); and        iii. adjusting solution composition by blending with other solutions (U.S. Pat. No. 6,663,689).        
U.S. Pat. No. 6,755,891 to CESL relates to processing arsenical copper materials and is an extension of earlier patents using the CESL process, which includes pressure leaching under medium temperature (150° C.) conditions to form elemental sulphur in the presence of significant levels of chloride. While the concentration of chloride is not listed in the patent, a concentration of 12 g/L Cl is indicated in other publications.
Despite the above-mentioned teachings relating to treating arsenical copper materials, or other arsenic-containing materials, there remains a need for a process to effectively remove arsenic from copper containing feeds while producing environmentally stable arsenic solids from the process.